Pulping lignocellulose in continuous pressurized batch digesters

Abstract

Lignocellulose is pulped batchwise by introducing it and the white liquor in predetermined quantity, together with a variable quantity of black liquor, into a continuously steam pressurized digester. When the cooking cycle has been completed, the pulp product is withdrawn. The time intervals of feed material introduction and pulp product withdrawal are correlated to insure that all of the lignocellulose charged to the digester is cooked uniformly, as by withdrawing the pulp product over a time interval which is substantially equal to the feed time. For economy and efficiency of operation, the digester preferably is operated in conjunction with one or more companion digesters to which it is connected. The first digester then is blown while a second digester is charged, making possible transfer of hot pressurized black liquor from the former to the latter.

Having thus described my invention in preferred embodiments, I claim: 
     
       1. The process for the batch production of cellulosic pulps by the chemical digestion of pieces of lignocellulose which comprises: (a) providing a pressurizeable batch reaction vessel defining a vertical path along which the lignocellulose progresses from top to bottom,   (b) pressurizing the vessel with steam to substantially the predetermined digestion pressure and temperature,   (c) while maintaining the vessel at substantially digestion pressure and temperature charging the vessel with flows of presteamed lignocellulose pieces and both hot fresh and hot spent digestion liquors for a predetermined charging period,   (d) the fresh liquor flow containing digestion chemicals in the predetermined amount being charged to the vessel at a substantially constant rate commensurate with the rate of charging the lignocellulose pieces,   (e) the spent liquor flow being charged to the vessel at a variable rate sufficient to maintain during the charging period a substantially constant rate of liquor rise within the vessel and to keep the lignocellulose pieces substantially submerged in the liquor, thereby compensating for variations in water extraneously introduced into the digester from steam condensation and lignocellulose water content,   (f) digesting the lignocellulose in the vessel for the predetermined digestion period at the digestion pressure and temperature,   (g) when the lignocellulose in the lower part of the digester has been digested for said digestion period, continuously withdrawing the resulting pulp from the bottom of the vessel over a withdrawing period substantially equal to the vessel-charging period, thereby insuring subjection of the top and bottom portions of the lignocellulose charge to equal durations of pulping treatment within the vessel, and   (h) while withdrawing the pulp from the vessel, diffusing cold spent digestion liquor from a stock thereof into a plurality of vertically and radially spaced diffusion zones in the bottom of the vessel at a diffusion rate predetermined to pass the displacement liquor radially through the zones during a time interval substantially equal to the time required for vertical travel of the pulp through the vertical zones.   
     
     
       2. The process of claim 1 including the step of introducing steam into the upper portion of the vessel for establishing therein a steam zone and passing the fresh liquor through the steam zone for elevating its temperature to substantially the digestion temperature. 
     
     
       3. The process of claim 1 including the step of providing at least two pressurizeable reaction vessels, charging one of the vessels while discharging the other, and passing in measured flow the hot spent digestion liquor from the discharging vessel to the charging vessel as required to maintain a substantially constant rate of liquor rise in the charging vessel.